Converter circuit



Dec. 29, 1936. E. w. HEROLD 2,066,038

CONVERTER CIRCUIT Filed Aug. 10, 1935 I ITlga 1 MM 7 m 2 2 7 DETECTOR 4 3 5mm 5: :E V J- SOURCE 55% "F I l C a I l i I" Y K 70 A 1 6 +750l +250v I SOL/RC5 3 C l I l i M l w l 5 12 400- Q mess/v7 X it com mm? g I Q g g, 200- a PRIOR Q co/v'v' Emma to x Q i, 700- I, m I u 0567101708 PEAK V0175 ME INVENTOR.

EDWARD W. HEROLD BY 7% Q MW ATTORNE tube.

Patented Dec. 29, 1936 CONVERTER CIRCUIT Edward W. Herold, Verona, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 10, 1935, Serial No. 35,593

3 Claims.

My present invention relates to frequency converters adapted for use in signaling systems, and more particularly to a novel and improved type of frequency converter network adapted for use in superheterodyne receivers.

Frequency converter networks of a type utilizing electronic coupling are well known at the present time. One of the types in common usage at the present day employs as a converter tube a device known as a pentagrid converter tube. In

such a converter the signal and local oscillation grids are separated by a positive screening grid. The output electrode, or plate, of the converter tube has developed in its circuit the opcrating intermediate frequency energy. Such converter tubes often have circuits associated therewith, in addition to the usual signal circuits, which render them operative as a composite first detector1ocal oscillator network. In other arrangements the converter tube of the multi-grid type may be used with an independent local oscillator.

In the past at least two conventional methods of coupling the local oscillator, for example, of the triode type, to a pentagrid mixer tube have been used. In one of these methods, the local oscillator grid of the mixer tube, is maintained at the same direct current and alternating current potentials as the grid of\ the local oscillator In such a circuit, grid current is drawn by both the converter oscillator grid, and the local oscillator tube grid, and the effect of such grid current flow is to automatically bias these grids by the drop in the grid leak of the local oscillator tube. Accordingly, if the oscillator peak voltage changes, then the direct current bias changes a proportional amount, and, therefore, shifts the working point on the converter oscillation grid characteristic. Since it is found that the highest conversion gain in a pentagrid converter tube results from operation at an oscillation grid direct current bias which is nearly constant, this method of coupling the oscillator tube to the converter tube is not desirable.

5 A second conventional method of coupling between a local oscillator tube and a converter tube involves the utilization of a separate leak and condenser for the converter oscillation grid. The same condition exists in this case as described above. An additional disadvantage of this second the main objects of the present invention to overcome the disadvantages enumerated heretofore in the coupling of a pentagrid converter tube to a local oscillator tube, and to accomplish this with a novel coupling arrangement involving the 5 direct connection of the local oscillator tube grid and the converter oscillation grid, the direct current bias on the converter oscillation grid being partially fixed by a voltage drop developed in the cathode circuit of the converter tube. 10

Another important object of this invention is to provide a coupling circuit between a converter tube of the pentagrid type and a local oscillator tube wherein the oscillation grid of the converter and the grid of the oscillator tube are 15 directly connected together so that the alternating current voltage on the converter oscillation grid and the local oscillator grid is the same, but wherein the direct current bias on the converter oscillation grid is never less than that due to the voltage drop across the biasing resistor in the converter cathode circuit and wherein the local oscillator grid current supplies an additional bias by the voltage drop in the local oscillator grid leak. 25

Still another object of the invention is to provide a frequency changer network for a superheterodyne receiver, the network being of a type comprising a converter tube of the pentagrid type and a local oscillator, and wherein the 'cou- 30 pling circuit between the local oscillator tube grid and the converter oscillation grid is such that the network has substantially a two to one advantage over conventional coupling arrangements when the oscillator voltage is low, and for 5 higher oscillator voltages there is considerably less variation of gain with oscillator voltage.

Other objects of the invention are to improve generally the efliciency of operation of frequency changer networks for superheterodyne receivers, ,40 and more especially to provide frequency changer networks which are not only reliable in operation and productive of higher conversion gains than conventional circuits of the same type, but are economically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in -par ticularity in the appended claims; the inve-n- 0 tion itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organiza- .55

I metallic tube envelope. velope of tube 3 in. Fig. l is shown as grounded.

7 The circuit arrangementshown in Fig. 1 comprises a signal source I which is adapted to feed signals of a desired signal frequency range into the tunable input circuit 2 of the first detector network. detector comprises a tube 3 of the so-called pentagrid converter type. 7 shown in the drawing is of the 6L7 type, and is a tube Whose electrodes are disposed within a For this reason the en- While theconstruction of tube 3 is not'a part of the present invention, it is pointed out that the tube comprisesthe usual cathode, signal input gridandoutput plate. The-signal grid is connectedto the high alternating potential side of the tunable signal input circuit-2, and the oathode is connected to ground through the usual cathode bias resistor R which is shunted by by pass condenser '6. As is well'known to those skilled in the art, the voltage drop across resistor R furnishes the normal'operating bias for the signal grid of tube 3, andthe signal grid returns to ground through the conventional AVC (automatic volume control) circuit. 'It is not believed necessary to show the details of such. AVC circuit since those skilled in the art are'fully aware of the construction and 'opera- 7 tion of such a circuit.

It is believed sufficient for the purposes of the present application to point out that for signals of an intensity less than a'predetermined value,

. the voltage drop'across resistor R furnishes the sole operating bias. for the signal grid. When the signals impressed onxcilfcuit 2 increase above that value then the AVG circuit goesinto operation and acts to increase the negative bias on the signal grid of "tube 3; The condenser C is connected betweenthe low alternating potential side of input circuit 2 and the cathode side of bias resistor R, and this condenser furnishes a path for the signal currents.

Between thesignal grid andplate of tube 3 there are disposed four grids, the third grid from the. cathode functioning as the oscillation grid,

and the second and fourth grids being main-7 tained at a positive potential of approximately 150 volts, and functioning to electrostatically screen the signal grid from the oscillation grid. The fifth grid is disposed between the fourth, or positive, grid and the output plate; and it will be noted that the fifth grid is connected to the cathode of tube 3 within the tube envelope. The plate of tube 3 may have a positive potential of approximately 250 volts impressed thereon, and the operating intermediate frequency energy is derivedfrom resonant circuit 4 which is connected to the plate. The circuit 4 is-coupled to the following resonant circuit 5, the

latter also being tuned to the operating intermediate frequency. The networks following circuit 5 may comprise one or more stages of intermediate frequency amplification followed by a demodulator, audio network and a reproducer. The AVC rectifiermay be the second detector itself, or, if desired, a separate rectifier may be used for this purpose. By way of illustration,

It is to be understood that'the'first The specific type tube it is pointedout that the automatic volume control circuit shown in Fig. 1 of application Serial No. 29,014, filed June 29, 1935, by Paul F. G.

Holst may be used in the receiving system shown inFig. 1 of the'present application.

The signal source .-I which feeds the tunable signal input circuit 2 of the first detector tube '3 may comprise the usual signal collector feeding one or more stages ;of tunable radio frequency amplification. "The local oscillator is a triode whosecathode is connected to the grounded side of the bias resistor R, and between whose controlgrid and cathode is connected the tunable local oscillation circuit 6. The control grid of oscillator tube 1- is connected to one side of the tunable circuit 6 through the condenser 8,

' and the grid leak resistor 9 is connected between the cathode and control grid of oscillator tube 1. The oscillation grid l3 of tube 3 is connected by lead Hi to the control grid sideof leak resistor9.

coupled, through coil H, to thetunable circuit 6, and a positivepotential of approximately 150 volts may be applied to the plate of oscillator tube 1.

The dotted lines I2 designate the usual me I chanical uni-control device employed to vary the rotors of the variable tuning condensers 2' and 6' of the tunable circuits 2 and 6 respectively.

The locally produced oscillation voltage is impressed on the third grid of converter tube 3, and it will be observed that this oscillation grid l3 and the control grid of oscillatortube I are at the same alternating current potential, due to the direct connection In. In so far as direct current potentials are concerned, the direct current bias on oscillation grid l3 is due to the voltage drop across resistor R, and the voltage drop produced across the leak resistor 9 by the oscillator grid current. Hence, it will be seen that the direct current voltage on the oscillation grid I3 is never less than'that due to the voltage drop Y across bias resistor R. Typical operating volt-' ages for certain of the electrodes of the frequency changer network have been given. The signal gridmay be operated with a bias of 6 volts, and a direct current bias of approximately 20 volts may, be utilized on'theroscillation grid l3.

It may be'pointed out that the peak oscillator.

voltage applied to the oscillation grid I3 is approximately .25'volts. and that the conversion conductance is approximately 325 micromhos.

InFig. 2 is shown graphicallythe practical significance of the advantage obtained'by the. use of the present invention overthe conventional coupling circuits described heretofore. In Fig. 2 there is shown a comparison between two curves secured by plotting CONVERSION CONDUCT- ANCE against OSCILLATOR PEAK VOLTS. The dotted line curve shows the curve of conversion conductance against peak oscillator voltage secured by prior converters using typical operating voltages, and this dotted line curve is compared with the full line curve designating the operation of the present invention. It will be observed from Fig. 2 that the converter circuit shown in Fig. 1 has almost a'two to one advantage over conventional arrangements when the oscillator voltage is low. For higher oscillator voltages. there is considerably less variation of gain with oscillator voltage whenthe present circuit is used. J This is a decided advantage in receiver design in as much as oscillator voltages commonly vary two to one over the tuning range while it is preferable to keep the gain approximately constant. It will '75 20 The plate of oscillator tube! is regeneratively V now be seen that by virtue of the present invention the local oscillator network is coupled to the converter network in such a manner that the highest conversion gain is secured, and that its efficient operation results because of the maintenance of the direct current bias on the oscillation grid 13 at a value which is nearly constant. It is pointed out that the present coupling arrangement is equally applicable to other types of converter networks wherein a separate oscillator tube is used.

While I have indicated and described a system for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination, in a frequency changer network, a mixer tube of the type including a cathode, a signal grid, a plate, an oscillation grid, and means for electrostatically screening the signal grid from the oscillation grid, a local oscillator network including a tube having at least a cathode, control grid and plate, the oscillator plate being regeneratively coupled to the input circuit of the oscillator tube to provide local oscillations, a grid leak resistor connected between the control grid and cathode of the oscillator tube, a direct current connection between the oscillation grid of the mixer tube and the oscillator tube control grid, an impedance in the cathode circuit of said mixer tube for developing a predetermined direct current voltage thereacross from the space current flow therethrough, said leak resistor and impedance being connected in series whereby the bias on said oscillation grid is the sum of said voltage and the direct voltage drop across the leak resistor.

2. In combination, in a frequency changer network, a mixer tube of the type including a cathode, a signal grid, a plate, an oscillation grid, and means for electrostatically screening the signal grid from the oscillation grid, a local oscillator network including a tube having at least a cathode, control grid and plate, the oscillator plate being regeneratively coupled to the input circuit of the oscillator tube to provide local oscillations, a grid leak resistor connected between the control grid and cathode of the oscillator tube, a direct current connection between the oscillation grid of the mixer tube and the oscillator tube control grid, an impedance in the cathode circuit of said mixer tube for developing a predetermined direct current voltage thereacross from the space current flow therethrough, said leak resistor and impedance being connected in series whereby the bias on said oscillation grid is the sum of said voltage andthe direct voltage drop across the leak resistor, and a direct current connection between the signal grid of said mixer tube and the junction of said impedance and leak resistor.

3. In combination, in a frequency changer network, a mixer tube of the type including a cathode, a signal grid, a plate, an oscillation grid, and means for electrostatically screening the signal grid from the oscillation grid, a local oscillator network including a tube having at least a cathode, control grid and plate, the oscillator plate being regeneratively coupled to the input circuit of the oscillator tube to provide local oscillations, a grid leak resistor connected between the control grid and cathode of the oscillator tube, a direct current connection between the oscillation grid of the mixer tube and the oscillator tube control grid side of the leak resistor, an impedance in the cathode circuit of said mixer tube for developing a predetermined direct current voltage thereacross from the space current flow therethrough, said leak resistor and impedance being connected in series whereby the bias on said oscillation grid is the sum of said voltage and the direct voltage drop across the leak resistor, a tunable signal input circuit connected between the signal grid and cathode of the mixer tube, a tunable oscillation circuit connected between the cathode and control grid of the oscillator tube and in shunt with said grid leak resistor, and a condenser connected between one side of said oscillator tube tunable circuit and the oscillator tube control grid.

EDWARD W. HEROLD. 

